Remove the use of MAIL_REPLY_TO

This was to set the Reply-To: mail header in messages sent from the
application. This header is what would be used when replying to the
message.

We don't actually want to set this header at all, people should not be
replying to these emails. They come from noreply@dnsandmx.com and any
replies should be sent there which in turn just goes to /dev/null.

So remove all uses of MAIL_REPLY_TO

Signed-off-by: Andrew Clayton <andrew@dnsandmx.com>
config: Set the envelope sender in the example config

Emails from the application come from noreply@dnsandmx.com. Replies to
these emails just end up in /dev/null. What we do want is any error
messages to go somewhere they'll be seen. So we set the envelope sender
(-f) on the sendmail command to a real address.

Show this in the example config file.

Signed-off-by: Andrew Clayton <andrew@dnsandmx.com>